A senior member of Syria's opposition coalition said on Friday opponents of President Bashar al-Assad had smuggled samples from victims of an alleged chemical weapons attack out of Syria for testing by experts.

"We took them and sent them outside Syria," Syrian National Coalition Secretary General Badr Jamous told Reuters in Istanbul, but declined to say where the samples had been sent.

President Bashar al-Assad is under increasing international pressure to allow U.N. inspectors access to the rebel-held site of Wednesday's reported mass poisoning, which if confirmed would be the world's deadliest chemical attack in decades.

The opposition coalition said it would ensure the safety of U.N. chemical weapons inspectors in areas of Syria it controls.

"We will ensure the safety of the U.N. team ... It is critical that those inspectors get there within 48 hours," coalition spokesman Khaled Saleh told a news conference.

Saleh's comments were an apparent rebuttal of suggestions from Moscow earlier that the opposition was preventing an objective investigation into allegations that the Syrian government carried out a chemical weapons attack.

"Much needed signals from the opposition, including its readiness to guarantee the safety and effective work of U.N. experts on territory controlled by militants, unfortunately are not forthcoming," the Foreign Ministry said in a statement.

"It is directly preventing an objective investigation into the possible use of chemical weapons in Syria."

Syrian activists have said they are trying to get body tissue samples from victims of the attack near Damascus to the team of U.N. inspectors staying in a hotel a few miles away.

The U.N. experts have been in Syria since Sunday to investigate three previous alleged chemical attacks in the country dating from months ago.

In what it said was a preliminary report into Wednesday's attack based on multiple sources, including a source within Assad's forces and testimony from opposition activists, the opposition coalition said 16 missiles were launched in an initial assault shortly after 2:30 a.m. (2330 GMT).

Local residents reported missiles falling in the Zamalka neighborhood on the eastern outskirts of the capital, far from the front lines, and hundreds of people died in their homes as exposure to the gas spread, the report said, noting that not all of the missiles were carrying chemical warheads.

It said a second district came under missile attack around ten minutes later and that shortly after 4:00 a.m. another 18 missiles were launched towards a southern suburb. Citing activists, it said four of the missiles to hit that neighborhood were loaded with chemical weapons.

"Medical reports show the victims exhibited symptoms consistent with exposure to sarin gas," the report said.

"However, we cannot definitively ascertain the exact nature of the materials and the deployment method used to target civilians in the Damascus suburbs," it said, and urged an immediate visit by the U.N. inspectors.